Why is brushing teeth important in preventing dental caries?How do microorganisms contribute to caries?

BRUSHING IS MOST IMPORTANT FOR KEEPING THE GUMS HEALTHY. CARIES (CAVITIES) ARE THE RESULT OF DE-CALCIFICATION OF TOOTH ENAMEL AND/OR DENTIN. SINCE CARIES ALSO OCCUR BETWEEN TEETH, FLOSSING IS THE ONLY WAY TO AVOID THESE TYPES OF CAVITIES.
ONCE THE MICROORGANISMS (BACTERIA) BECOME STUCK IN THE NARROW PORTION OF THE TOOTH (PIT OR FISSURE), THEN THOSE BACTERIA DE-CALCIFY THE UNDERLYING ENAMEL (THROUGH ACIDIC BYPRODUCTS), AND CAUSE THE CAVITY.

Caries?

I think you mean CAVITIES. Here's how it works: Everyone's mouth produces saliva, which is full of useful bacteria that helps you start to digest food. The problem is when sugars mix with these bacteria. If left untreated, the combination of sugars and bacteria makes two things: (1) Plaque, which causes problems with your gums, and (2) Acids that eat into and through the enamel of your teeth, making holes better known as cavities. Brushing your teeth helps remove the plaque and acids that cause gum disease and cavities. By the way, toothpaste is NOT necessary when brushing teeth, but if you don't use toothpaste you should submerge the bristles of your toothbrush in Listerine or some type of antiseptic mouthwash to kill any bacteria that may be left on your brush after you brush your teeth.